The outlets are divided into two categories, one is called FM outlet or FM point, and the other is called AM outlet or AM point. There is another kind of dots generated by hybrid screening. This kind of dots use AM dots as the skeleton and FM dots as decoration. The dots have different shapes and slightly random positions, and have the characteristics of amplitude modulation and FM dots. The advantage of hybrid screening is that there is no problem of dot angle and direction. The low frequency components are reduced to reduce the graininess of the picture, and the high frequency components are reduced to improve the printing stability. Therefore, the printing suitability of mixed dots is better than amplitude modulation and FM dots. This section focuses on FM and AM outlets.
The FM network outlet is formed by using a computer with the cooperation of hardware and software. The distribution of dots in space is irregular, that is, randomly distributed. There are two basic types of FM screening, one is that the area of ​​each dot remains unchanged, depending on the density of the dots, that is, the frequency of the dots in the spatial distribution, so that the light and dark levels of the image on the manuscript are reproduced on the printed matter (Figure 1-4). This is what is usually referred to as an FM network point, also known as a first-level FM network point; another is that the size and spatial distribution frequency of the network point are changing, and it is called a second-level FM network point.
a- FM outlets b- AM outlets Figure 1-4 Classification of outlets
(1) FM outlets
The main advantages of FM outlets are:
(1) No moiré will occur. Since the FM dots are randomly arranged, the moire is completely eliminated in theory (FM screening has no concept of mesh angle).
(2) Since FM screening does not rely on the change of dot size to adjust the amount of ink, few or even one laser spot can be used to form dots. The dots can be made very small, making the printed matter more fine.
(3) Generally speaking, FM dots rarely expand in printing.
(4) It is easy to realize high-fidelity printing. Since it is not limited by the printing screen angle, more than four-color printing can be used to increase the color reproduction range.
Despite the above advantages of FM screening, there are also some shortcomings, and some of the shortcomings are still difficult to solve under the current printing conditions, which limits the use of FM screening. The main problems are:
(1) The loss of outlets is serious. Due to the small screen dots, it almost reaches the limit of printing and printing, so it is easy to lose the dots during the printing and printing process, resulting in a large loss of image hierarchy.
(2) Strict requirements on printing conditions. Due to the small dots, the precision of the printing press is high, and the conditions of the ink and ink balance are not easy to grasp, and it will also cause the image quality to deteriorate (FM screening is very suitable for waterless offset printing).
(4) The difficulty of quality control increases. Since FM screening is a new screening method, the traditional methods of observing dots and judging colors are no longer applicable, and the rules of dot expansion are different. Therefore, it is difficult for current operators to control printing quality. .
(5) The calculation of the screen is large, and the output speed is slow.
FM screening is a brand-new process, which can print fine products and can also be used for low-precision newspaper printing, so it has attracted widespread attention in the printing industry.
(2) Amplitude modulation outlets
Amplitude modulation dots are dots formed by electronic screens using a camera screen or an electronic color separation machine with a dot generator using a laser. At present, due to the limitations of operator quality, process conditions, and precision of offset presses in China, amplitude modulation dots are still widely used in offset printing plates.
1. The three elements of amplitude modulation outlets
In offset printing plate making, there are three factors to determine the amplitude modulation dots, namely the percentage of dots, the number of dots and the angle of dots.
(1) The percentage of dots refers to the percentage of the area occupied by dots per unit area, that is, the area coverage rate of dots. In China, the percentage of outlets is used to express the percentage of outlets.
Table 1-1 The algorithm for changing the size of the dot area
100%
90%
80%
70%
60%
50%
40%
30%
20%
10%
Field
Jiucheng
80%
70%
60%
Fifty percent
Four percent
30%
20%
10%
The percentage of dots controls the area covered by ink in the unit area of ​​the paper, so that part of the light is absorbed and partly reflected. For example, in a halftone dot, 10% of the area of ​​the paper is covered with ink to absorb light, and 90% of the paper reflects light. In the same way, 50% of the dots mean that 50% of the area of ​​the paper is covered by ink, and the light absorbed and reflected by the paper accounts for half. On the other hand, 90% of the dots are covered by ink on a unit area of ​​paper, which absorbs light, and 10% of the paper reflects light. In contrast, 10% of the dots absorb less light and reflect more light, while 90% of the dots absorb more light and reflect less light. The size of the dots is divided into ten levels, plus small white dots and small black dots, so that the dots are divided into ten and twenty-two levels. The dots between these 10 to 22 levels can show the different brightness of the picture.
For the identification of the dot size, usually 10 to 25 times magnifying glass is used for visual identification in actual work. The specific body rules are:
a If it can accommodate three dots of the same size between two dots, it is called Yicheng dot.
b If it can accommodate two dots of the same size between two dots, it is called a 20% dot.
c. If it can accommodate 1 dot of the same size between two dots, it is called 30% dot.
d. If it can accommodate 1 dot of the same size between two dots, it is called Sicheng dot.
e Black and white, half, two dots can accommodate a dot of the same size, called 50% dots.
Table 1-2 Calculation of the number of outlets
The number of outlets below 50% and the number of outlets above 50% can be complemented accordingly, that is, 40% of outlets are complementary to 60% of outlets; 30% of outlets are complementary to 70% of outlets; 20% of outlets are complementary to 80% of outlets; 90% of the outlets are complementary. That is, starting from 60% of the dots, the white dots can be judged by how many white dots of the same size can be accommodated. The identification is shown in Figure 1-5.
Figure 1-5 Dot spacing diagram
The dot shape is divided into common dot shape and special dot shape. Common dot shapes (square dots, round dots, chain dots) The screen is the dot shape commonly used in production at present. Special dot shapes (mother-child dots, triple dots, ripple dots, concentric circular dots, etc.) are dot shapes specially designed to obtain special artistic effects to improve the tone of the image (Figure 1-6). Now, with the development of computer technology, the screening technology is also constantly updated, for example, art screening, three-dimensional screening, physical-like screening technology, etc., are constantly being applied in offset platemaking.
Figure 1-6 Types of outlets
(2) The number of dots is also called the number of screens or the frequency of screens. It refers to the number of parallel lines per unit length, that is, the number of unidirectional transparent parallel lines per inch.
Because 1 inch = 2.54cm, the two types of network cable system, imperial and metric, can be converted to each other (Table 1-3).
The thinner the network cable, the more dots can be accommodated in a unit area, and the higher the dot line number, the faster the number of dots can be accommodated in a unit area. The increase in the number of dot lines greatly enriches the expression level of the printed matter and enhances the effect of tone reproduction.
Conversely, if the number of dot lines is reduced, the level expressed by the screen will be weakened, and the effect of tone reproduction will also be reduced. However, the choice of the number of dot lines mainly depends on the type of printed matter, the type of paper and the surface condition.
Table 1-3 Intercalculation of the number of screen lines
Line / inch
60
75
80
100
120
133
150
175
200
Line / cm
twenty four
30
32
40
48
54
60
70
80
Table 1-4 Network cable for different printed materials
Dot thickness
Print category
Line of sight
Paper
80 ～ 100 lines / inch
Full posters, posters, movie posters, etc.
Far away
Trick sticker
100 ～ 133 lines / inch
Folk New Year pictures, posters, educational wall charts, etc.
Far away
Offset paper
150 ～ 175 lines / inch
Monthly calendar, postcards, pictorials, albums, book covers, etc.
Closer
Coated paper
175 ～ 200 lines / inch
Fine picture album, ancient painting reproduction, exquisite technology illustration
Closer
Advanced coated paper, advanced whiteboard paper
(3) Dot angle The dot angle generally refers to the angle between the dot arrangement line (mesh line) and the horizontal line (reference line).
Offset printing uses overprinting plates with different dots to overprint and reproduce the color of the original. For four-color overprinted prints, if the four printing plates have the same dot angle and the corresponding dots on each plate can accurately overlap during printing, then the best results will be obtained, but this is impossible. Usually there is a certain angle difference for the dot angles of the four printing plates (Figure 1-7). Both theoretical calculations and human visual experience have found that when the difference in dot angle of the printing plate is 30 °, the visual effect of the printed matter is the best, the human eye can hardly see the moire formed by the interference, and the image is uniform and harmonious. When the dot angle difference is 22.5 °, the visual effect of the printed matter is not as good as the former, but it also looks soft and pleasing. When the dot angle difference is 15 °, the visual effect of the printed matter is the worst, and it looks extremely uncomfortable, so in practice, try to adopt a 30 ° or 22.5 ° dot angle difference, and avoid a 15 ° dot angle difference. However, within the range of 90 °, only three colors can be arranged with an angle difference of 30 °, and another color has to use an angle difference of 15 °. Since the yellow is the lightest, it is closest to the brightness of the white paper. The visual perception of cyan, magenta, and black inks is stronger than that of yellow, so cyan, magenta, and black are usually arranged at a 30 ° angle difference, and the yellow version is arranged at a 15 ° angle difference.
Figure 1-7 Angle of the network
In order to avoid the appearance of moire, through the exploration of theory and practice, the dot angle of multi-color printing is carefully arranged.
If the four printing plates are taken at four different angles of 90 °, 75 °, 45 °, and 15 °, respectively, the difference in dot angles between the second, third, and fourth plates is 30 °. Only the first and second plates The angle difference between the dots is 15 °. In terms of visual effects, the dots arranged at 45 ° have a dynamic beauty and look particularly comfortable. The dots arranged at 90 ° appear dull, and the dots arranged at 15 ° or 75 ° have an effect somewhere in between. In addition, in the angular arrangement, it must be considered that the strong color version (main color version) mostly adopts a 45 ° screen angle.
In the offset printing process, different forms of dots are used, that is, a certain percentage of dots, dot lines and dot angles are used to reproduce the tone of the original manuscript.
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